1. Field Of The Invention
This invention relates to a magnetic induction coil for use as a transformer or a small inductor in a stable power supply. In particular, the invention relates to a magnetic induction coil integrated into the substrate of a semiconductor device.
2. Discussion Of The Related Art
In the conventional stable power supply, e.g., a switching power source or a chopper control device, the usual method of construction involved fixing independent parts, such as a capacitor or the magnetic inductor of a transformer, on a printed substrate along with an independent semiconductor chip for switching or rectifying. Recently, however, the desire to reduce the size and increase application of these circuits has led to a new architecture known as a Hybrid IC. In such devices, semiconductor elements are fixed on a ceramic substrate as an ordinary chip, and, except for heavy power supplies, it is now possible to integrate all independent elements on the same chip with a control circuit.
In contrast with the rapid integration of the semiconductor elements, it has been difficult to reduce the size of the independent parts, particularly a magnetic induction element. This difficulty retards the downsizing of the entire device.
A conventional magnetic induction element may be reduced in size by raising the operating frequency of the device. Such an increase in operating frequency permits a decrease in the size of the induction element while affording the same reactance. However, at higher frequencies, e.g., above 1 MHz, operating frequency increases tend to have a deleterious effect on and unduly reduce the efficiency of these devices.
Utilizing semiconductor process technology, this problem is solved using a multilayer construction of thin film conductor and a magnetic thin film in a fine structural pattern so as to minimize, simultaneously, an IC circuit and the magnetic induction elements in one chip. Known constructions of thin film multilayer magnetic induction elements include a thin film conductor of spiral-type or zigzag-type coil interposed between a pair of magnetic thin films, and where lines of thin film conductor and magnetic thin film are braided as a fabric. In either construction, since the magnetic induction elements are formed in a small and flat shape under 10 mm in size and only several tens of micrometers thick, the elements can be integrated into one chip of the semiconductor device.
To realize further downsizing, the magnetic induction elements of the above-described thin film multilayer construction are formed in a fine structural pattern of a thin film conductor by use of semiconductor technology, and the construction also has the advantage of raising the frequency characteristics of the inductor so that the operating frequency is over 1 MHz. Because the coil is formed by a thin film conductor, however, the width of the conductor may be limited in order to form the fine structure for the pattern. Accordingly, the current capacity of the magnetic induction elements is also limited, thus limiting the current density in the coil.
In order to solve the above problem, the thickness of the thin film conductor should be increased. However, it is difficult during an etching treatment to form the height of a coil conductor larger than the relative width between lines of the thin film conductor. Moreover, when etched, it is difficult to laminate a thin film on the etched coil conductor due to the larger height, and radiation from the coil is increased.